Holography is well known and widely used in many commercial applications including display holography, security, advertising and holographic optical elements and gratings. A holographic image is produced when light is diffracted at a complex spatially varying diffraction grating, which, in the simplest terms, redirects the light towards the viewer in such a way as to give the illusion that the light is coming from a solid three-dimensional object.
This diffraction grating is produced by exposing a suitable photosensitive material to the optical interference pattern produced when two coherent light beams (usually produced by a laser) meet. The material records the variation in light intensity (as a variation in refractive index, absorption or thickness) and a corresponding diffraction grating results. If both light beams are simple collimated beams, the result will be a simple diffraction grating whose spatial period depends on the angle between the recording beams.
If the diffraction grating is illuminated with one of the recording beams (or a similar beam), it will diffract the light to reproduce the other recording beam. If one of the beams is a complex wave front coming from a three dimensional object, the recorded diffraction grating will have the property that it can reconstruct this wave front when illuminated with the other beam.
A wide variety of photosensitive materials are available which are suitable for recording holograms including photopolymers, silver halides, dichromated gelatin, photo resists, thermoplastics, photochromics and photo-refractive materials.
One of the biggest markets in the field of commercial holography is security holograms. Security holograms are commonly found on credit cards, bank notes, passports and concert tickets, for example and function as an authentication device.
In recent years, research has been carried out on the development of volume photopolymer holograms. The photopolymer phase reflection hologram is attractive for security holograms. Such a hologram is relatively thick by hologram standards, for example, of the order of tens of microns. This means that the diffraction efficiency can be very high which results in the production of eye-catching 3D images, visually quite different to the rainbow effect of the embossed hologram. A further feature of photopolymer reflection holograms is the capacity to angularly multiplex several holograms into one layer. In photopolymers with high refractive index modulation, this can produce a moving image effect. Even a small number of multiplexed holograms can enable toggling between two static images, so that text or warnings can be visible in conjunction with the holographic image. A further and advantageous characteristic of photopolymers is the broad range of wavelength sensitivity which enables several colour components in the hologram.
Another area of application of holograms is in sensors and sensing. Holograms that can change their optical properties when exposed to a change in their environment have been developed as holographic sensors. Holograms that are sensitive to a particular chemical substance such as heavy metal ions, alcohol in water, proton concentration or physical conditions such as humidity, temperature and pressure, for example, have been developed previously.
A holographic sensor that responded to pressure was reported in [C. R. Lowe, J. Blyth, and A. P. James, “Interrogation of a sensor,” 2006]. An emulsion consisting of acrylamide:methacrylamide (2:1, v/v) and a crosslinker methylenebisacrylamide (5 mol %) was deposited on a substrate to create a film, which was then polymerised by a free radical polymerisation. Using silver-halide chemistry, a hologram was recorded while the substrate was soaking in a water bath using a frequency doubled Nd:YAG laser (λ=532 nm). The resulting hologram was sandwiched using another transparent substrate and pressure was applied onto the holograms using a pair of G-clamps. The pressure of the clamps on the hologram resulted in a contraction in the volume of the hologram, thus causing the diffraction signal to blue-shift by a total of 3 nm. While the above publication discusses the principle of operation of a pressure sensitive hologram, a shift of 3 nm in wavelength is not large enough to produce a visible change in the colour of the image reconstructed from the hologram, and implies that the sensitivity of the reported material to pressure is not large enough for pressure-sensing applications.
Despite developments in the field of holographic recording materials, there remains a need for improved holographic recording compositions for pressure sensing.
The present invention is directed towards providing improved formulations for the preparation of holographic recording materials and to the improved performance of photosensitive holographic recording materials for applications, such as pressure sensing, for example.